The technical portion of an optician's profession consists in mounting a pair of ophthalmic lenses in a frame selected by a wearer. Such mounting comprises four main operations:                reading the outlines of the bezels in the rims of the frame selected by the wearer, i.e. the outlines of the grooves that run around the inside of each of the rims of the frame, said reading providing an image of the shape of the final outline that is to be presented by each ophthalmic lens once it has been shaped;        centering each lens, i.e. determining the position that each lens is to occupy in the frame so as to be properly centered facing the pupil of the wearer's eye, in such a manner as to enable the lens to act appropriately in performing the optical function for which it has been designed;        blocking each lens, which consists in depositing a blocking member on each lens at a position that is deduced from defined centering parameters; and        shaping each lens, which consists in machining or cutting out the lens to match the outline felt on the frame, with this being performed while the lens is held by means of the blocking member.        
In the context of the present invention, attention is paid to the second operation known as centering. Specifically, for the optician, this consists in defining the position to be occupied by the final outline relative to the optical frame of reference of said lens (typically relating to its usual marks or to its optical centering point), in such a manner that the lens is properly positioned facing the pupil of the wearer's eye so as to perform as well as possible the optical function for which it has been designed.
In a first stage, this operation consists in centering the final outline of the lens on the centering point of the lens so that once centered the line of gaze of the wearer passes through the centering point of the lens.
In a second stage, this operation then consists in orienting the final outline of the lens about the centering point of the lens so that once oriented, the reference axis of the lens (typically its cylinder axis or its horizontal axis) presents a desired angle relative to the horizon line of the frame.
The Applicant has found that in spite of the care given to performing those four operations, it can happen that ophthalmic lenses are not correctly positioned relative to the frame, thereby leading to visual discomfort for the wearer.